Real-time systems must be able to ensure temporally determinate execution of real-time tasks at run-time. By temporal accuracy, we refer to the timing accuracy with which the execution of a task can be started at a predetermined time. Temporally determinate execution of tasks on modern high performance processors is becoming more and more difficult because of the techniques used by these processors to boost their average performance. This report describes the experiments we have conducted to measure the temporal accuracy that can be achieved with the Pentium Pro processor. We present the results of these experiments and analyze these results to highlight the limitations of temporally determinate execution of programs on modern high performance processor architectures.